Unit 4: The 5 Parameters of Pulsed Sound

Vocabulary flashcards defining pulsed sound parameters, formulas, and relationships from Unit 4 ultrasound physics notes.

Transmit Time

The time when sound is being actively produced by the transducer, also referred to as on-time or talking time.

Receive Time

The time when sound is off and the machine listens to process returning echoes and create image information, also referred to as off-time or listening time.

Pulse

A collection of cycles that work as a unit and travel together, having a clear beginning and end.

Pulse Duration (PD)

The actual time it takes to complete the transmit portion of one pulse, measured in microseconds ($\mu s$). Average ultrasound values range from $0.3$ to $2\,\mu s$.

Spatial Pulse Length (SPL)

The distance that a single pulse occupies in space from start to finish, measured in millimeters ($mm$). Average ultrasound values range from $0.1$ to $1\,mm$ in soft tissue.

Pulse Repetition Period (PRP)

The time from the start of one pulse to the start of the next pulse, which includes both transmit time (Pulse Duration) and receive time.

Pulse Repetition Frequency (PRF)

The number of pulses that an ultrasound system transmits into the body per second, typically measured in kilohertz ($kHz$).

Duty Factor (DF)

The percentage or fraction of time that the ultrasound machine is transmitting sound energy into the body.

Maximum Imaging Depth

The maximum distance into the body that the system images, indicated by the scale at the bottom of the field of view.

Pulse Duration Formula (using Period)

$$\text{Pulse Duration}(\mu s) = \text{\# cycles} \times T(\mu s)$$

Pulse Duration Formula (using Frequency)

$$\text{Pulse Duration}(\mu s) = \frac{\text{\# cycles}}{f(\text{MHz})}$$

Spatial Pulse Length Formula

$$\text{Spatial Pulse Length}(mm) = \text{\# cycles} \times \lambda(mm)$$

Pulse Repetition Period Formula (using Depth)

$$\text{PRP}(\mu s) = 13\,\mu s \times \text{depth}(cm)$$

Pulse Repetition Frequency Formula (using Depth)

$$\text{PRF}(\text{Hz}) = \frac{77{,}000\text{ cm/s}}{\text{depth}(cm)}$$

Duty Factor Formula

$$\text{Duty Factor}(\%) = \frac{\text{Pulse Duration}(\mu s)}{\text{Pulse Repetition Period}(\mu s)} \times 100$$

Reciprocal Relationship of PRP and PRF

$\text{PRP} \times \text{PRF} = 1$, meaning as one increases, the other decreases.

Depth-Dependent Parameters

The parameters determined by the system that change when the sonographer adjusts the imaging depth: Pulse Repetition Period (PRP), Pulse Repetition Frequency (PRF), and Duty Factor (DF).

Depth-Independent Pulsed Parameters

Pulsed wave parameters determined by the system/medium that do not change when imaging depth is altered: Pulse Duration (PD) and Spatial Pulse Length (SPL).

Shallow Imaging Characteristics

Imaging with less depth, resulting in less off-time, shorter PRP, higher PRF, and higher Duty Factor.

Deep Imaging Characteristics

Imaging with greater depth, resulting in more off-time, longer PRP, lower PRF, and lower Duty Factor.

Clinical Cycles per Pulse

In clinical grayscale (b-mode) imaging, pulses typically contain $2\text{ to }4$ cycles. Doppler imaging uses more cycles per pulse.

Continuous Wave Duty Factor

$100\%$, because continuous wave ultrasound transmits sound constantly without any receive or off-time.

Grayscale 2D Imaging Duty Factor

Typically less than $1\%$, meaning the machine spends more than $99\%$ of its time listening/receiving echoes.